The present invention relates to a device in the drying section of a paper machine for running the web as a closed draw, which device operates in connection with a drying fabric arranged in such a way that one cylinder/several cylinders or roll/rolls of the drying section is/are outside the drying fabric loop and the other cylinder/cylinders or roll/rolls is/are inside the drying fabric loop so that the web passes from one cylinder or roll to the other cylinder or roll over the whole distance supported by said fabric.
Regarding the state of art, reference is made to Finnish patent applications Nos. 761953 and 771056, which correspond to U.S. Pat. Nos. 4,183,148 and 4,202,113 respectively, assigned to applicant's assignee, to Finnish Pat. No. 54954 corresponding to U.S. Pat. No. 4,172,007 also assigned to applicant's assignee, and to the Finnish patent application No. 780833 of J. M. Voith GmbH (related DE-GM No. 7 708 630).
The Finnish patent application No. 761953 corresponding to U.S. Pat. No. 4,183,148 discloses a procedure for providing a closed paper web draw in the beginning of the multicylinder dryer that comprises two rows of drying cylinders preferably one on top of the other. The first part of one of the rows, preferably the top row, is enclosed within the first wire or felt that supports the wire as it passes from one cylinder row to the other so that the web is, when being in association with one cylinder row, on top of said felt or wire, and when being in association with the cylinders of the other cylinder row, between the felt or wire and the surface of the cylinders of the row in question. In the procedure depicted above, the essentially new feature is that, in order to hold the web on the surface of the cylinders of that cylinder row whose cylinders are enclosed within the first wire or felt, at this point, another wire or other wires, preferably of a coarse-meshed type and essentially of the width of the web, is/are passed on the web, in order to press the web on the sector on which the web touches said cylinders, at the most, and that, in order to prevent friction between the wire or felt and the wire and the consequent dusting of the wire, the angular speed of the second wire or felt in relation with center points of the drying cylinders in question, is arranged to be automatically adjusted to be the same as the angular speed of the web that runs on these cylinders.
The Finnish patent application No. 771056 corresponding to U.S. Pat. No. 4,202,113 mentioned above discloses a procedure in the drying section of a paper machine for guiding the web as a closed draw, in which procedure, in the beginning of the drying section, particularly in its first group of drying cylinders, a drying wire or fabric is used which is arranged in such a way that the cylinders of one row are outside the fabric loop and the cylinders of the other row are within the fabric loop so that the web runs zig-zag from one cylinder row to the other at all times supported by the same fabric, from the beginning to the end of the drying cylinder group that is equipped with said fabric. An essentially new feature in this known method is that, at least at some of the cylinders at which the web is outside the fabric, a pressure-difference is, via the grooved surfaces of these cylinders, applied to the web so as to make the pressure outside the web higher as compared with the pressure in the grooves of the cylinder surface, particularly for preventing the web from separating from the fabric and for ensuring the continuous operation of the paper machine.
In said Finnish Pat. No. 54954 corresponding to U.S. Pat. No. 4,172,007 a procedure is disclosed for ensuring the draw of the web from the press section to the drying section, in which procedure the main new feature is that between the drying cylinder group and the press section there is a separate lead drying cylinder that does actually not belong to this group; that, for holding the web in contact with said bottom drying belt as it runs around the top cylinders of said special cylinder group, an overhead drying belt, essentially of the width of the web, is passed onto the web in order to press the web against the bottom drying belt over a sector that is essentially narrower than the sector at which the bottom drying belt supporting the web covers the top cylinders; and that said overhead drying belt can be made to touch the lead drying cylinder by means of an adjustably positionable lead roll at least for the phase during which the web, when the paper machine is started, is transferred from the press section to the drying section.
In the above-mentioned Finnish patent application No. 780833 of J. M. Voith GmbH a paper machine drying section is disclosed, wherein in the wedge-shaped space between the drying belt and drying cylinder at the entry side and/or at the exit side there is an air lock essentially reaching over the whole length of the drying cylinder in such a way that the penetration of air into said wedge-shaped spaces is prevented.
With the known device described above it is not possible to completely prevent the web from separating the drying belt and consequent web ruptures.
In the above-mentioned procedures and devices, in which the web follows the drying fabric in such a way that, at the bottom cylinders or similar of the drying cylinder group, the web will be the outermost element, the web tends, due to air currents and centrifugal forces, to separate from the drying fabric. The risk of such separation mainly increases with the square of the web speed. Another important reason why the web tends to separate from the drying fabric is that the drying cylinder and the drying fabric induce an air flow that tends to separate the web from the drying fabric. Another drawback related with these facts is that air enters between the drying fabric and the drying cylinders which reduces thermal conduction. At current web speeds it has been possible to eliminate these problems by using suitable felt. As the machine speeds have increased, it has not entirely been possible to avoid said phenomena.